As a conventional method for reducing NOx contained in combustion exhaust gases of, for example, diesel engines, it had been considered effective to add a reducing agent to exhaust gas, and bring the resulting exhaust gas into contact with a NOx catalyst, such as a metallosilicate system having a molecular sieve structure, as disclosed in Japanese laid-open Utility Model Publication No. 4-54926.
It is most convenient to use diesel fuel or light oil of the diesel engine, as the reducing agent, for improving NOx reducing capability of NOx catalyst. In a known NOx reduction system as schematically shown in FIG. 3, a NOx converter 03 is mounted in an exhaust passage 01 of a diesel engine, with a NOx catalyst 02 accommodated in the converter 03, and a fuel injection device 04 is provided at a certain position of the exhaust passage located upstream of the NOx converter 03, so that diesel fuel can be directly injected into the exhaust passage to be added to the exhaust gas. With this known method, however, sufficiently high NOx reduction efficiency cannot be obtained, as shown in the graph of FIG. 4 (curve B).
In the graph of FIG. 4, the vertical axis represents the NOx reduction efficiency (%), and the horizontal axis represents the temperature of a test gas having substantially the same composition as the exhaust gas of an actual diesel engine. Thus, FIG. 4 shows the results of a test of measuring NOx reduction capability, in which test he flow rate of the test gas (SV) was equal to 4000 h.sup.-1, and diesel fuel or light oil was added to the test gas at a concentration of 3000 PPM.
The exhaust gas temperature of diesel engines for trucks is generally in the range of 100 to 200.degree. C. during idling operations, and about 500.degree. C. during full-throttle operations. From the standpoint of the relationship between the operation state of the engine and the NOx concentration in the exhaust gas, it is highly desirable or ideal to provide a NOx reducing characteristic as indicated by line A of FIG. 4 consisting of three segments that form the upper part of a trapezoid. The above-described method, in which diesel fuel is directly injected into the exhaust gas, however, provides a NOx reduction characteristic as indicated by curve B in FIG. 4, in which the values of the NOx reduction efficiency are far lower than those of the target performance line A. More specifically, the NOx reduction efficiency in a narrow range of the exhaust gas temperature around 400.degree. C. is only slightly higher than 30%, which is unsatisfactory for practical use. It is to be noted that the range of the test gas temperature that is higher than 500.degree. C. corresponds to an overload operating state that rarely occurs during operations of the engine.
In an exhaust gas purifying system as disclosed in Japanese laid-open Utility Model Publication No. 4-54926, diesel fuel is thermally decomposed and added to exhaust gas, which is then supplied to a NOx catalytic converter to improve the NOx reduction efficiency. FIG. 5 schematically shows the construction of the system as disclosed in the above-identified Utility Model Publication No. 4-54926. The system of FIG. 5 includes a four-cycle diesel engine 05 for a motor vehicle, an in-line fuel injector pump 06 that supplies fuel to a fuel injection nozzle 07 provided for each cylinder of the engine 05 through a corresponding fuel supply pipe 08, a feed pump 09 that supplies fuel in a fuel tank to the fuel injector pump 06 through a fuel supply pipe 010, an exhaust passage 011 including an exhaust manifold 012 of the diesel engine 05, and a NOx catalytic converter 013 disposed in the exhaust passage 011. A NOx catalyst 014 containing metallosilicate is accommodated in the catalytic converter 013.
A thermal decomposition device 015 is connected to a portion of the exhaust passage 011 located upstream of the NOx catalytic converter 013. The thermal decomposition device 015 includes a decomposition chamber 017 surrounded by an electric heater 016, a fuel injection valve 018 that injects diesel fuel into the decomposition chamber 017, and an exhaust gas supply passage 020 that supplies exhaust gas in the exhaust passage 011 to the decomposition chamber 017 through a flow rate control valve 019. A fuel supply branch pipe 021 is also provided through which the diesel fuel is supplied from the fuel supply pipe 010 to the fuel injection valve 018.
In the system as disclosed in the above-indicated Japanese Utility Model Publication No. 4-54926, exhaust gas supplied from the exhaust supply passage 020 to the decomposition chamber 017 is heated to a high temperature in the vicinity of 1000.degree. C. by the electric heater 016, and the diesel fuel injected from the fuel injection valve 018 is thermally decomposed under the high-temperature atmosphere, to provide unsaturated lower hydrocarbon for use in NOx reduction. Owing to the presence of this reducing HC, the NOx reduction capability of the NOx catalyst 014 is supposed to be improved.
In the meantime, the content of oxygen in exhaust gas emitted from the diesel engine 05 is generally about 18 to 20% during idling, and as small as about 3 to 5% during full-throttle operations. Since the content of oxygen is particularly small during half-throttle and full-throttle operations, in which a large amount of NOx is contained in exhaust gas, only a small amount of HC is produced for effectively reducing NOx even if the temperature of the decomposition chamber 017 is raised to as high as almost 1000.degree. C., resulting in insufficient NOx reduction efficiency of the NOx catalyst 014. Also, the atmosphere used for thermal decomposition needs to be heated to as high as about 1000.degree. C., which is disadvantageous in terms of increased electric power consumed by the electric heater 016.
In another exhaust gas purifying system as disclosed in Japanese laid-open Patent Publication No. 6-212952, the thermal decomposition device 015 as disclosed in the above-indicated Utility Model Publication No. 4-54926 is replaced by a fuel reforming device using a suitable catalyst.
The fuel-reforming device serves to reform diesel fuel or light oil into a lower-molecular-weight compound at about 280.degree. C. by using a catalyst for reforming diesel fuel, and produce oxygen-containing lower hydrocarbon by supplying the air or exhaust gas to the fuel.
Although the power consumed by the electric heater can be reduced by use of the fuel reforming device, the catalyst for reforming diesel fuel is expensive, and thus the use of the catalyst results in increases in the size and cost of the system.
The present invention was developed to solve the problems encountered in the known systems as described above, namely, the system in which diesel fuel is directly injected into the exhaust passage located upstream of the NOx catalytic converter, the system as disclosed in Japanese laid-open Utility Model Publication No. 4-54926 wherein diesel fuel is injected into a high-temperature atmosphere (exhaust gas) heated to about 1000.degree. C., and thermally decomposed to produce unsaturated lower hydrocarbon, which is then added as reducing HC into the exhaust passage upstream of the NOx catalytic converter, and the system, in which diesel fuel is reformed into a low-molecular-weight compound, using a catalyst for reforming diesel fuel, and the air or exhaust is supplied to produce oxygen-containing lower hydrocarbon, which is added as NOx reducing HC into the exhaust passage upstream of the NOx catalytic converter. It is, therefore, an object of the present invention to provide a NOx reduction system for combustion exhaust gas, which provides higher NOx reducing capability than the known systems, and simple in structure and available at a lower cost.